How important is CCD size?

I started UWP just few years ago with compact camera Oly XZ-1. With small strobes and Inon lens I can do images sharp in corners at f around 4. Then the same lens I attached to Sony RX-100. To obtain similar sharpness and constant Iso=100 I have to close aperture up to f 6 or 8. Now I started to think about changing for DSLR. In tests of lenses I see that wide lens has to be closed to f 14 or 16 .

If I want to get sharp images in the corners, what is the advantage of bigger CCD like FullFrame ??

When I am looking for new lens I always check the resolution and sharpness diagrams and lenses like Tokina 10-17 dosent looks so great.

Even if the question sounds stupid or basic, please be patient to give answer. I am really in doubt.

Soft corners is not primarily connected to sensor size. It normally shows up for wide angle photography. I don't do any WA but there are several discussions on wetpixel on what combinations of lens, dome, diopter give the best results. Do a search in the DSLR forum and you will probably find some. You should be able to shoot decent lenses with much larger aperture, at least F4-F5.6.

Going from a compact to interchangeable lens camera has more to do with the ability to select the optimal lens for a task and more control and higher performance of those cameras. Sensor size discussions can bring out passionate debates with valid points on both sides. In the end it depends one what you need and can afford.

I am mainly interested in WAL or CF WAL in dark water. I really doesn't like soft corners, for me it looks like mistake. I saw few test of Tokina 10-17 lens and in all of them the f value was mentioned rather close to 8.

"All dome ports sharpest in the corners at F11, and usually fairly sharp at F8, and not too bad at F5.6"*

I am just a bit disappointed, because when I changed to a bit bigger sensor (1/1.63" to 1') then I learned (thank You Interceptor ) that I need stronger strobes to keep the same sharpness (with smaller aperture ) of the images, and now I am worry even more because I am considering to switch for a DSLR

I'd say sensor size and sensitivity are two important factors. The "soft edges" are a function of matching a specific lens to a specific camera. Some lens have better results than other. Many zoom lenses offer a great value, but simply don't act well in all aspects, at all focal lengths.

For WA, strobes will only do you so much. Depending on water column clarity, speed of subject, distance, and strobe power (guide number), you still need to reduce the distance of the camera to the subject. Strobes can't light an entire scene 50 feet away (and you'd have backscatter issues). Often, they're used as fill flash in the foreground, or snooted to highlight a foreground object, and let ambient light govern the background.

I agree with dpaustex. Having a larger sensor means better image quality and the lenses are more suited to their use, so tend to have slightly better quality. However with any interchangeable lens camera whether it be a cropped frame or full frame, it is necessary to match the lens to the correct port. Most underwater housing companies offer many different dome ports in sizes from 4" up to 9", and various different extensions for them. If the wrong length extension is used, then the port will be too close or too far from the lens.

Generally, for rectilinear wide angle lenses (those which are not fisheye), then the larger the dome the better the corner sharpness. However, they tend not to have anywhere near as sharp corners as fisheye lenses such as the tokina 10-17. Often rectilinear lenses require magnifying diopters on them to get the corners sharper which requires trial and error to find the right one.

The Tokina 10-17 is a very good lens for cropped sensor cameras, and one that is used by most of the professional uw photographers. If you are not happy with the corner sharpness, I would suggest that either the wrong port is used (there are often 4" ports especially for that lens) or you are too picky. With a fisheye lens, it allows you get much closer to the subject and hence images tend to look sharper as there is more detail and the strobes light up more.